A bandgap voltage reference circuit can be used, e.g., to provide a substantially constant reference voltage for a circuit that operates in an environment where the temperature fluctuates. A conventional bandgap voltage reference circuit typically adds a voltage complimentary to absolute temperature (VCTAT) to a voltage proportional to absolute temperature (VPTAT) to produce a bandgap reference output voltage (VGO). The VCTAT is typically a simple diode voltage, also referred to as a base to emitter voltage drop, forward voltage drop, or simply VBE. Such a diode voltage is typically provided by a diode connected transistor (i.e., a transistor having its base and collector connected together). The VPTAT is typically derived from a difference between the VBEs of two transistors having different emitter areas and/or currents, and thus, operating at different current densities. For example, the ΔVBE quantity can be from an 1:8 ratioing of transistor sizes (i.e., emitter areas) running at equal currents. This results in VT*ln 8≈53 mV, where VT is the thermal voltage, which is ≈25.7 mV at room temperature (25° C. or 298° K). More specifically, VT=kT/q, where k is the Boltzmann constant, q is the charge on the electron, and T is the operating temperature in degrees Kelvin.
Where a bandgap voltage output (VGO)≈1.2 V, a VPTAT of ≈0.5 V can be added to the VBE of ≈0.7V. The VPTAT≈0.5 V can be achieved by producing a ΔVBE≈53 mV, using a pair of transistors having an 1:8 ratio of emitter areas, and using an amplifier having a gain factor≈9, i.e., 53 mV*9≈0.5V. In other words, 53 mV can be gained up by a factor of ≈9 to achieve a VPTAT≈0.5 V. This, however, also results in all the noises associated with the ΔVBE also being gained up by a factor of ≈9, which is undesirable. Such noises can include, e.g., transistor and resistor noises.